A lens is described in Japanese Examined Patent Application (Kokoku) H7-95143 that is well-known as a large numerical aperture objective lens. This lens has an F-number of about 1.85 with a visual field of about 12°, and thus provides a bright telescopic lens which favorably corrects distortion and other aberrations. This lens is well suited for use as an objective lens for a photographic camera or a video camera. However, an objective lens for a night vision optical device that uses a photoelectron amplifier tube must have an even smaller F-number in order to provide a sufficiently bright image. Moreover, an objective lens for a night vision optical device must generally produce a large negative distortion. The reason is that the eyepiece in a night vision optical device generally produces a positive distortion aberration, and correction of such distortion in the eyepiece alone is very difficult. Therefore, a technique is generally used wherein a large negative distortion of about −4% to about −9% is generated in the objective lens for a night vision optical device in order to cancel the positive distortion of the eyepiece. In this manner, favorable correction of distortion of the night vision optical device is provided.
Because the objective lens of a night vision optical device does not require a large back focus, and a compact objective lens is desired, a telescopic lens would normally be suitable, except for the requirement that the objective lens produce a large negative distortion, as described above. Although it would appear that an objective lens having a large negative distortion could be readily designed by simply arranging a surface having a strongly negative refractive power near the object side of a lens system or near the image side of a lens system, such an arrangement makes it difficult, in a compact arrangement of lens elements, to favorably correct both coma and astigmatism.